Air filter of polyurethane mesh containing carbon adsorbent

ABSTRACT

Air-cleaning filter element prepared by applying an adhesive agent such as a synthetic rubber latex to at least a pair of three-dimensionally mesh-structured elastic-flexible webs, disposing evenly an adsorbent material, such as activated carbon, in a size range of 0.5 mm to 10 mm in diameter between said pair of elastic-flexible webs and pressing the same elastic-flexible webs together as superimposed. The air-cleaning filter element is free from such a deterioration of the adsorbent material as caused by an adhesive and any significant fluid pressure drop on passage of fluid.

This invention relates to an air-cleaning filter element prepared bybonding an adsorbent material to and between a plurality ofelastic-flexible webs with an adhesive agent, which has the inherentadsorptive performance of said adsorbent.

Heretofore, as filter materials for air cleaner and other uses,adsorbent materials, e.g. activated carbon have been commonly used aspacked in plate-shaped or sigzag-shaped filter housings but theadsorbents tend to give dust on vibration or friction of the housings inthe course of transportation or use, thereby soiling the housings. Therealso are generated gaps in the carbon layer and the resultant"short-cuts" of the fluid to be treated resulted in the failure of thefilter to display its expected cleaning performance.

To overcome the above disadvantages, various methods have heretoforebeen proposed for molding an adsorbent with a binding agent to provide adimensionally stable adsorbent product. Recently, it has been proposedto place a granular adsorbent in a nonwoven fabric case and seal thecase to provide a sealed filter element, or to retain a side of anadsorbent-packed filter element with a spring member so as to preventgeneration of gaps.

However, the binding agents herefore used are disadvantageous in thatthe solvents contained in such binders tend to drastically reduce theadsorptive capacity of the adsorbent or cause a considerable pressuredrop on passage of the fluid. Or the filter housing is of necessitycomplicated in structure. Thus, the prior art devices have not providedfully satisfactory results.

In view of the foregoing disadvantages, the present inventors conductedintensive studies on the web to be used, the type of binder, moldingconditions, etc. and have perfected a method of producing anair-cleaning filter element which is free from a deterioration of theadsorbent material and any significant fluid pressure drop on passage ofthe fluid.

This invention is therefore concerned with an air-cleaning filterelement prepared by applying an adhesive agent to and between at least apair of three-dimensionally mesh-structured elastic-flexible webs,disposing an adsorbent material, such as activated carbon, in a sizerange of 0.5 to 10 mm, preferably 1 to 5 mm in diameter, between saidelastic-flexible webs and pressing the webs together as superimposed.

The mesh-structured elastic-flexible web to be employed has athree-dimensional skeletal structure which can be attained by selectivehydrolysis of a polyurethane foam and which contains 9 to 31 air spaces(hereinafter referred to as cells) per 25 linear millimeters, the numberor density of such cells being selected according to the grain size ofthe activated carbon or other absorbent employed. Thus, for example, aweb having 9 cells/25 linear mm is preferred for the adsorbent having agrain size of 4 to 5 mm and a web having 31 cells/25 mm for non-uniformirregularly shaped adsorbent grains of the order of 1 mm.

As regards the thickness of said mesh-structured elastic-flexible web, aweb material of the described type may be sliced or otherwise processedto a thickness suitable for the grain size of carbon to be bondedthereto and the intended application. An appropriate thickness can beselected within the range of 3 to 10 mm.

The adsorbent employed according to this invention may for example beactivated carbon, active clay, acid clay, silica, alumina or any ofmolded or crushed metal oxide catalysts, or a mixture of two or more ofsuch materials, although activated carbon is particularly advantageous.The activated carbon may be one based on coal, petroleum oildistillation residue, wood, fruit shell or the like, provided only thatit has a BET specific surface area of 500 to 2000 m² /gram. Theadsorbent is normally used as cylindrical, spherical or irregular-shapedgrains from 0.5 mm to 10 mm, preferably from 1 mm to 5 mm in diameter.

The adhesive agent employed according to this invention may be of anytype only if it is able to secure said adsorbent in position on saidmesh-structured elastic and flexible web. Thus, solvent-sensitiveadhesives, heat-sensitive adhesives and reaction-sensitive adhesives maybe mentioned as examples, although solvent-sensitive adhesives areparticularly desirable. As examples of solvent sensitive adhesives whichare preferably used in the present invention there are mentioned varioussynthetic rubber latices which are aqueous emulsion of synthetic rubberof 0.03μ to 1.5μ in particle size. The examples of the solid bodycomponent of the latices are described below.

(1) Butadiene polymers, or copolymers from butadiene and styrene,styrene derivatives, acrylonitrile, methacrylonitrile, isoprene,isobutylene, etc.

(2) Copolymers from isoprene and styrene, styrene derivatives, etc.

(3) Chloroprene polymers, or copolymers from chloroprene and styrene,styrene derivatives, acrylonitrile, isoprene, etc.

(4) Copolymers from acrylates and styrene, styrene derivatives, vinylchloride, vinyl acetate, acrylonitrile and methacrylate.

(5) Methacrylonitrile polymers, and copolymers from methacrylonitrileand styrene, etc.

(6) Vinyl acetate polymers and vinyl chloride polymers.

Also usable are the above latices having been carboxy-modified orsuitably modified in other ways.

These latices may be an aqueous emulsion obtained mainly from emulsionpolymerization, and are used, with the resin solid content adjusted to20 to 50% and preferably, 35 to 45%.

The present air-cleaning filter element can be prepared by the steps ofapplying said adhesive agent to mesh-structured elastic-flexible webs,disposing the adsorbent over the coated surfaces of said webs andbonding the webs together by the applicaation of pressure.

The application of the adhesive agent to elastic-flexible webs can beaccomplished by direct brushing, roller or calender-coating,spray-coating or any other suitable procldure. It is, however, moredesirable to take the steps of coating a release paper with the adhesiveby brushing, roller-coating, spray-coating or the like, pressing thepaper against the elastic-flexible web in face-to-face relationship and,then, peeling off the release paper to let the adhesive be transferredfrom the release paper to the surface of the elastic-flexible web.

While the amount of adhesive should vary with its type, the preferredamount as the solid matter thereof is 5 g. to 550 g. per square meter ofweb, preferably, 100 g. to 500 g. or, for still better results, 250 g.to 500 g. per m².

With a smaller amount of adhesive, the carbon cannot be effectivelylocked in position on the web, and with an excess of adhesive, the opencells of webs are covered with the adhesive to cause an undesirablygreat pressure drop on passage of fluid.

When the adhesive contains a solvent, the solvent should be evaporatedoff prior to deposition of the adsorbent, for otherwise the solvent willbe adsorbed on the adhesive to interfere with the latter's adsorptiveability. When the application of adhesive to the elastic-flexible web isperformed through a release paper, the evaporation of solvent may takeplace after the application of adhesive to the release paper and beforethe pressing of the paper against the web, or after joining, i.e. beforepeeling-off or thereafter. The solvent evaporation may be accomplishedeither by allowng the material to stand at ambient temperature andpressure of, if necessary, at elevated temperature and/or pressure. Thedeposition of the adsorbent on the elastic-flexible web may beaccomplished by a suitable procedure, e.g. by hand or by mechnicalvibrations. While the deposition amount of adsorbent is optionalaccording to the intended application, it is normally 200 to 2500 gramsper square meter of the web.

The bonding of carbon to the adhesive-coated webs can be accomplished bysupporting the webs between wood, plastic, metal or other plates andapplying an external load of 0.01 to 0.02 kg/cm².

There are cases in which the edges of thus-formed filter element areheat-sealed.

The air-cleaning filter element according to the present invention maybe one prepared by using not less than 3 sheets of elastic-flexiblewebs. For example, in case of using 5 sheets of webs (A), (B), (C), (D)and (E) are superimposed in the order mentioned, adsorbent is fixed ineach space between webs, which are neighboring each other, i.e. thespace between webs (A) and (B), the space between webs (B) and (C), thespace between webs (C) and (D), and the space between webs (D) and (E).The adsorbent in each space of the webs is bonded to two webs existingboth sides of the adsorbent.

The folowing examples are further illustrative of this invention.

EXAMPLE 1

An elastic-flexible web was applied by brushes with an adhesive agent atan application rate as resin solids of 350 g/m², followed by scatteringuniformly activated carbon on the surface at a rate of 3 l/m², thenoverlapped by the elastic-flexible web applied with an adhesive agent inthe similar manner, and put between plates to be allowed to stand atambient temperature for 10 hours under load of 0.01 kg/cm² applied.Among the materials hereby used, elastic-flexible web was one having asize [15 cm×30 cm×5 mm (thickness)] and a standard cell number of 9pieces/25 mm, while the activated carbon was cylindrical-formed oneshowing 1180 m² /g in BET specific surface area, 4 to 6 mesh in grainsize (4 mm in average particle size), and 450 g/l in packing density,with the adhesive agent as indicated in Table 1.

                  TABLE 1                                                         ______________________________________                                        Sample No.                                                                              Type and solid content of latices used                                                                (%)                                         ______________________________________                                        1         Carboxy-modified SBR    47                                          2         Carboxy-modified MBR    48                                          3         Polyvinyl acetate       50                                          4         NBR                     45                                          5         Copolymer from acrylic acid ester                                                                     45                                          ______________________________________                                    

With the filter materials produced in this way, performance testing wascarried out in accordance with the following methods, along with theobtained results shown in Table 1:

1. Pressure loss

The filter element was out into a disc 66 mm across and placed in acolumn of the same inside diameter. Dry air at room temperature wasintroduced into the column at a linear flow rate of 0.5 to 2 m/sec. andthe linear flow rate of the effluent air was measured with an electronicanemometer for pressure drop determination.

2. Acetone adsorption rate

The filter element was cut into a disc 66 mm across and fitted with anadsorption column. A gas containing about 1.0 vol. % of acetone in a gasholder was circulated to the adsorption column at a linear flow rate of15 cm/sec. and the adsorption rate was determined from the time when theacetone concentration had reached one-tenth of its initialconcentration.

3. Acetone equilibrial adsorption

The filter element was cut into a disc-shaped piece 66 mm across and, inaccordance with JIS K-1474, the equilibrial adsorption of acetone withan initial concentration of 1000 ppm was determined.

                  TABLE 2                                                         ______________________________________                                                                         Acetone equili-                                                               brium adsorp-                                Sample Pressure loss                                                                             Acetone adsorption                                                                          tion, g/100cm.sup.2                          No.    mm aq.lm/sec.                                                                             rate, min.    filter                                       ______________________________________                                        1      0.5         8.0           4.3                                          2      0.7         7.5           4.3                                          3      0.9         11.0          4.0                                          4      0.7         8.0           4.2                                          5      0.8         9.0           4.1                                          Unfabri-                                                                             0.6         7.0           4.3                                          cated                                                                         ______________________________________                                    

EXAMPLE 2

By varying the application amount of a synthetic rubber latex(carboxy-modified SBR latex), in the same procedure as described inExample 1, filter materials were produced to carry out the performancetesting as mentioned in Example 1, to obtain the results shown in Table3.

                  TABLE 3                                                         ______________________________________                                                                           Acetone                                           Application                                                                             Pressure  Acetone equilibrium                                Sample amount of loss mmaq.                                                                              adsorption                                                                            adsorption,                                No.    latex, g/m.sup.2                                                                        lm/sec.   rate, min.                                                                            g/100cm.sup.2 filter                       ______________________________________                                        6      300       0.7       8.0     4.2                                        7      400       0.7       7.5     4.3                                        8      500       1.2       12.0    4.2                                        9      600       25.6      21.0    4.0                                        Unfabri-                                                                             --        0.6       7.0     4.3                                        cated                                                                         ______________________________________                                    

Reference Example 1

In accordance with the same procedure as described in Example 1, and bycrushing the cylindrical-formed activated carbon as used in Example 1 toa uniform grain size of 24 to 48 mesh (average particle size of 0.42mm), by the use of a carboxy-modified SBR latex as an adhesive agent andan elastic-flexible web of 50 pieces/25 mm in standard cell number as amesh-structured elastic-flexible one, with the application amount of theadhesive agent of 400 g (solids)/m², a filter was produced to determinethe pressure drop and equilibrium adsorption quantity for acetone, asbeing indicated in the following:

Pressure loss;

(at a flow rate of 1 m/sec. as a linear flow rate) 75.6 mm aq.

Acetone equilibrial adsorption; (2.0 g/100 cm² -filter)

The drawings show the embodiments of the air filter of the presentinvention. FIG. 1 is a partially exploded perspective view of thethree-dimensionally mesh-structured elastic-flexible web. FIG. 2 andFIG. 3 are partially exploded perspective views of the air filter of thepresent invention, respectively. FIG. 2 is directed to the present airfilter comprising two sheets of the three-dimensionally mesh-structuredelastic-flexible web and one layer of activated carbon existing betweenthe two sheets of web. FIG. 3 is directed to the present air filtercomprising five sheets of web and four layers of activated carbon, eachlayer existing between one sheet of web and another sheet of web.

1: three-dimensionally mesh-structured elastic-flexible web.

2: activated carbon

3: the present air filter

What is claimed is:
 1. Air cleaning filter element which comprises atleast two sheets of three dimensional mesh-structured elastic-flexiblewebs prepared by selective hydrolysis of a polyurethane foam having 3 mmto 10 mm thickness as superimposed, said webs containing 9 to 31 airspaces per 25 linear millimeters, and an activated carbon adsorbenthaving a BET specific surface area of 500 to 2000 m² /g and a grain sizeof 1 to 5 mm in diameter, said adsorbent being deposited in an amountbetween 200 and 2500 grams per square meter and bonded to and betweenthe webs with an amount of adhesive agent.
 2. Air-cleaning filterelement as claimed in claim 1, wherein the adhesive agent is asolvent-sensitive adhesive.
 3. Air-cleaning filter element as claimed inclaim 1, wherein the adhesive agent is a synthetic rubber latex. 4.Air-cleaning filter element as claimed in claim 1, wherein the adhesiveagent is a synthetic rubber latex whose solid body component isbutadiene polymer or a copolymer from butadiene and styrene, styrenederivatives, acrylonitrile, methacrylonitrile, isoprene or isobutylene.5. Air-cleaning filter element as claimed in claim 1, wherein theadhesive agent is used in an amount ranging from 100 g. to 500 g. persquare meter of web.
 6. Air-cleaning filter element as claimed in claim1, wherein the adhesive agent is used in an amount ranging from 250 g.to 500 g. per square meter of web.
 7. A method for preparing anair-cleaning filter element, said air cleaning filter element comprisingat least two sheets of three dimensional mesh-structured elasticflexible webs prepared by selective hydrolysis of a polyurethane foamhaving 3 mm to 10 mm thickness as superimposed, said webs containing 9to 31 air spaces per 25 linear millimeters, and an activated carbonadsorbent having a BET specific surface area of 500 to 2000 mg² /g and agrain size of 1 to 5 mm in diameter, said adsorbent being deposited inan amount between 200 and 2500 grams per square meter and bonded to andbetween the webs with an amount of adhesive agent, said methodcomprising the steps of:(i) applying an adhesive agent to and between atleast two sheets of said three dimensional mesh-structuredelastic-flexible webs; (ii) disposing said adsorbent between saidelastic-flexible webs; and (iii) pressing said elastic-flexible webstogether as superimposed.